1. Field of the Invention
The present invention relates to a thin film magnetic head which has a small flying height above a disc, as well as to a process for production thereof. More particularly, the present invention relates to a thin film magnetic head in which the width, groove depth and tapered angle of each rail at the surface side of said magnetic head to face a magnetic disc are formed to provide a high accuracy and at a high efficiency and which can give a small flying height stably and also can prevent head crush, as well as to a process for production thereof.
The present invention relates also to a plasma etching process using an etching gas. More particularly, the present invention relates to a process for etching using an etching gas that can process a material having a small etching rate (e.g., a ceramic or a high dielectric) in a short time, at a high accuracy, safely and easily.
2. Prior Art
In order to increase the recording density of a thin film magnetic head (hereinafter referred to simply as a magnetic head), it is essential to reduce and stabilize the flying height of the magnetic head. For achievement thereof to develop a magnetic head allowing for high-density recording, a particularly important task is to form, on a magnetic head, rail(s) which can minimize the variation in flying height of the magnetic head, caused by the difference in circumferential speed of a disc between the inner and outer circumferences of the disc.
Description is made on the formation of rail(s) by referring to FIG. 29 and FIG. 30.
FIGS. 29A and 29B are drawings showing the shapes of magnetic heads and processes for production thereof; FIGS. 30A to 30C are schematic drawings for explaining the flying state of a magnetic head; FIG. 31 is a graph showing the relation between rail width and flying height; and FIG. 32 is a graph showing the relation between rail groove depth and flying height.
In order to allow a magnetic head 1 to fly, there is utilized an air bearing slider preloaded by a suspension spring, such as shown in FIG. 30. The air bearing slider is a bearing mechanism consisting of an air layer between a magnetic disc 9 and the top surface of a magnetic head 1 facing the magnetic disc 9, i.e. the top surface of each rail 2 formed on a rail substrate 8 and, as shown in FIG. 30B, is formed by air which enters said layer from an air inlet 21. When the air, which has entered the layer, leaves the layer from an air outlet 22 at the end of the element portion 20 of the magnetic head 1, the resulting air current caused by viscosity resistance of air between the magnetic head 1 and the magnetic disc 9 imparts a flying force to each rail 2. In this case, the flying height 4 of the magnetic head 1 as shown in FIG. 30C is controlled by said flying force and the pressure of a spring 3 added to the magnetic head 1 from outside. The magnetic head 1 is in physical contact with the magnetic disc 9 when the magnetic disc 9 is in a stopped condition; when the magnetic disc 9 reaches a certain number of rotations per minute, an air bearing as mentioned above is formed, a flying force is generated, and the magnetic head 1 is separated from the magnetic disc 9 and keeps flying at a given flying height 4. With respect to the flying state of the magnetic head 1, its flying height 4 is smaller at the air outlet 22 than at the air inlet 21, as shown in FIG. 30B, and consequently the magnetic head 1 contacts with the magnetic disc 9 more easily at the air outlet 22 when the magnetic disc 9 is in rotation and also when stationary.
The shape of the portion of the magnetic head 1 at the air inlet 22 is desirably as smooth as possible to prevent, for example, the damage of the magnetic disc 9 or the element portion 20 of the magnetic head 1. To achieve such a shape efficiently for a large number of magnetic heads 1 is difficult using known techniques. As an approach, there is known a technique of chaffering the portion of a magnetic head 1 at the air outlet 22 by mechanical processing, specifically polishing.
Chaffering of each edge of rail top surface 2a (rail top surface 2a is hereinafter referred to simply as top surface 2a) has been conducted for the purposes of, for example, prevention of rail 2 sticking to magnetic disc 9, acceleration of flow of air onto top surface 2a (top surface 2a is a point of generation of the air dynamic pressure) at the start of flying of magnetic head 1, and prevention of magnetic disc 9 damage caused by the edge of top surface 2a and consequent destruction of recorded information. For this edge chaffering, there are proposed mechanical processing methods, for example, a method of polishing each rail 2 on a lapping sheet-attached rotating disc by allowing the rail 2 to repeat flying and contact with the disc in a stare similar to that experienced on a magnetic disc &gt;e.g. Japanese Patent Application Laid-Open No. 60-9656.
The flying height 4 depends upon the number of rotations per minute of magnetic disc 9, the dimension and shape of each rail 2 of magnetic head 1, the pressure of spring 3, etc. This flying height 4 must be minimized and moreover maintained stably in order for a magnetic disc device to allow for high-density recording. It is desirably 100 nm or less. Hence, a strict accuracy is required for the dimension of each rail 2 formed at the air bearing surface, the top surface of a magnetic head 1 which is to contact with a magnetic disc.
The relation between the flying height 4 and the width or groove depth of rail 2 is generally such as shown in FIG. 31 or 32, although it varies slightly depending upon the shape of rail 2. FIG. 31 shows a relation between rail width (.mu.m) and flying height 4 (.mu.m) when the rail groove depth (.mu.m) is constant. It is shown that the flying height 4 is larger when the rail width is larger. FIG. 32 shows a relation between rail groove depth and flying height 4 when the rail width is constant. It is shown that the flying height 4 is minimum when the rail groove depth is at a particular value and that the flying height 4 is larger when the rail groove depth is smaller or larger than the particular value. For example, in a case where the rail has a shape such as the non-linear rail 5 shown in FIG. 29B, the flying height 4 is minimum when the rail groove depth is 5-6 .mu.m (particular value). In this case, the design value of rail groove depth is set generally at 5-6.mu.m. With respect to the geometrical shape of rail 2 top surface, curved line shapes (e.g. a non-linear rail 5) are used practically to obtain a desired flying height 4 in an air bearing mechanism, or to minimize the adverse effects caused by the error in rail 2 formation or the error in formation of rail groove depth, or to minimize the change in flying height 4 by the difference in circumferential speed between the inner and outer circumferences of magnetic disc 9. Examples of other shapes are proposed in Japanese Patent Publication No. 5-8488 and Japanese Patent Application Laid-Open No. 4-276367.
For formation of a rail 2 which has a complicated shape as mentioned above and yet must have a dimension of high accuracy, a dry processing technique, particularly an ion milling technique is in use in place of the conventional mechanical processing using a whetstone as shown in FIG. 29A. The dry processing technique comprises forming a resist pattern matching the shape of a rail 2 to be formed, by photolithography, applying an ion beam 6 using the resist pattern as a mask, as shown in FIG. 29B, to etch a rail substrate 8, and finally removing the mask to form a rail 2.
In the dry processing technique, there is used, as the etching apparatus, an ion milling apparatus. The ion milling apparatus includes the following, for example:
(1) an ion milling apparatus wherein thermoelectrons are generated from a filament, a troidal movement is imparted to the thermoelectrons by an external magnetic field, an active gas is efficiently ionized by their troidal movement and thereby a plasma is generated, an active ion (an ion beam) is extracted from the plasma by an electrode, and processing (ion milling) is conducted with the ion beam; PA1 (2) an ion milling apparatus having an ECR (electron cyclotron resonance) ion source as shown in FIG. 5, wherein a microwave is generated by a microwave generator, electron cyclotron resonance is allowed to take place by the microwave and an external magnetic field, thereby an active gas is ionized efficiently to form a plasma, an active ion is extracted from the plasma by an electrode, and processing (ion milling) is conducted with the ion beam.
In processing a material by etching to form a fine pattern therein, processing by reactive ion etching (hereinafter referred to as RIE) or by ion milling has hitherto been carried out using, as the etching gas, Ar or a fluorine-containing compound gas such as CF.sub.4, CHF.sub.3 or the like.
In such processing, however, the ratio of the processing rate of material to be processed to the processing rate of mask, i.e., the selectivity, is as low as about 1.3. Consequently, a thick mask was required and, when the amount of processing was large as in the case of forming a rail groove of a magnetic head by the use of a carbon film as a mask, at least 10 and odd hours were required for the formation of said carbon film, causing the whole process to require a long time.
Further, when a thick mask was used, the mask caused a change in width during processing, which allowed the processed material to have a large dimensional shift and a large dimensional scatter and resultantly gave a low processing accuracy. Meanwhile, in the formation of a semiconductor or an optical element, the pattern width is very small although the processing amount is small. As a result, the dimensional shift of the mask used has a large influence on the processing accuracy. In order to solve these problems, it was important to use a mask material and a gas both capable of giving a selectivity as large as possible.
For the above reasons, various masks and gases to be used were studied, and CH.sub.4 (methane) gas or CH.sub.2 F.sub.2 (difluoromethane) gas has been used conventionally. It is known that, when these gases are used, a deposit appears on a mask such as carbon film, silicon film, resist, metal film or the like during etching and the selectivity becomes infinite.
When a conventional polishing technique is actually used to allow a magnetic head 1 to have a particular shape at the air outlet 22 by chaffering, it is currently difficult to conduct the chaffering efficiently because the refuse generated by polishing must be disposed and the element portion of the magnetic head may be deteriorated by the polishing solution used. In the case of, in particular, a magnetic head 1 having a protective film of very small thickness (5-30 nm) (not shown in the drawings) on the rail top surface for higher reliability, chaffering without damaging said protective film is extremely difficult because the protective film is too thin.
When mechanical polishing is used to conduct the chaffering of each edge of rail top surface 2a, it is difficult to achieve the chaffering uniformly for a large number of magnetic heads 1. This has been a problem for obtaining a magnetic head 1 capable of stably giving a low flying height.
Next, description is made on the mechanism of the above-mentioned conventional dry processing and the drawbacks thereof, by referring to FIGS. 33 to 37.
FIG. 33 is a drawing explaining the formation of a rail by ion milling using a patterned mask 7 made of a photoresist or the like; FIG. 34 is a graph showing the angular dependancies of ion milling rates; FIG. 35 is a drawing showing the change with time, of the sectional shape of a rail during rail formation; FIG. 36 is a graph showing the relation between rail groove depth and rail width; and FIG. 37 is a drawing showing a redeposition layer formed on each side of a rail.
In FIG. 33, a mask 7 is etched by an ion beam 6 and, simultaneously therewith, a rail substrate 8 is etched, whereby a rail 2 is formed. In this case, the etching rates of the mask 7 and the rail substrate 8 are both determined mainly by the angular dependancies of ion milling rates shown in FIG. 34. That is, as shown in FIG. 34, the ion milling rates of both the mask 7 and the rail substrate 8 increase gradually while the ion beam incident angle changes from 0.degree. to 40.degree., reach respective peaks between 40.degree. and 60.degree., and thereafter decrease sharply. For instance, the top surface of the mask 7 is processed at an ion milling rate when the ion beam incident angle is 0.degree. and the sides of the mask 7 are processed at an ion milling rate when the ion beam incident angle is about equal to a mask tapered angle .beta. shown in FIG. 33. Similarly, the rail substrate 8 which is to become the bottom of the rail 2 is processed at an ion milling rate when the ion beam incident angle is 0.degree., and the sides of the rail 2 are processed at an ion milling rate when the ion beam incident angle is equal to a rail tapered angle .alpha.. Practice of ion milling, however, teaches that the processing rate is not determined only by the angular dependence of ion milling rate.
The phenomenon of ion milling is complex because it includes not only the above-mentioned milling action per se, but also a phenomenon that the particles 12 sputtered by the ion beam 6 do not leave the material to be processed, but rather redeposit on the sides or bottom of the material, i.e., a redeposition phenomenon. While it is known that the redeposition of the sputtered particles 12 takes place because a certain proportion of the sputtered particles 12 are redeposited, it is very difficult to know the proportion quantitatively. This is why the ion milling phenomenon is complex.
FIG. 35 is a drawing showing the change with time, of rail sectional shape in ion milling. Anticipation of a final rail sectional shape 16, i.e., formation of a desired sectional shape at high reproducibility is currently very difficult even when the angular dependence of ion milling rate as shown in FIG. 34 is known beforehand, because there arise, for example, a phenomenon that the sectional shape of mask 7 (particularly, the mask tapered angle .beta.) and the rail tapered angle .alpha. change with time, a phenomenon of redeposition of sputtered particles 21, a variation in the manner of change of original mask 7 shape into its final shape, and a variation in ion milling conditions.
FIG. 36 is a graph showing the relation between rail groove depth and rail width when a rail 2 is formed using a conventional ion milling technique. As shown in FIG. 36, the rail width is smaller when the rail groove depth is larger, because the sides of the rail 2 are processed more. Currently, however, it is difficult to control the rail width at a desired accuracy, because there is currently no reliable technique by which the completion timing of processing is indicated when the rail groove depth has reached a certain value. Further, as is appreciated from FIG. 36, deviation of the rail groove depth from a desired value by 1 .mu.m results in deviation of the rail width by about 6 .mu.m. Because of this matter, it often occurs that the width of the rail formed deviates from the design value range even when the rail groove depth is processed in a desired range. This is one reason for low rail processing accuracy, low productivity of magnetic head 1, low stability of flying characteristics of magnetic head, etc.
In forming a rail 2 by ion milling, there is, besides the above-mentioned dimensional (e.g., rail width) accuracy problem, a further problem is that a redeposition layer 10 remains until the end of processing in some cases, as shown in the processing model of FIG. 33. Since this redeposition layer 10 is formed by deposition of sputtered rail substrate material, it cannot be removed by the use of an organic solvent, oxygen plasma ashing or the like and remains until the end of processing, for example, in a state as shown in FIG. 37, with the projected front end of the redeposition layer 10 protruding from the rail surface. This results in damage of magnetic disc 9 by the front end of redeposition layer 10 when the magnetic disc device is in operation and, in the worst case, results in destruction of recorded information.
The CH.sub.4 or CH.sub.2 F.sub.2 gas used in the conventional processing of various materials to form a fine pattern, is a combustible gas and dangerous. Hence, a large expenditure is necessary to implement safety measures for piping, etc. and the application of such a gas in a large-scale facility was difficult.
Further, the deposition film formed on a mask during etching has a large thickness and the thickness varies greatly. Therefore, the variation in dimensional shift after processing was large. Particularly in formation of fine groove for semiconductor or optical element, the mask thickness increases with the progress of processing; consequently the processing is conducted at high aspect ratios; the processed material redeposits on the sides of mask and groove; the sectional shape after processing becomes a triangle or a trapezoid; as a result, no desired sectional shape could be obtained.